Carbonic anhydrase II (CA-II) is a zinc enzyme that catalyzes the interconversion of CO2 and bicarbonate. The azide ion, which is a competitive inhibitor of bicarbonate dehydration in CA-II, binds at Zn2+ without compromising the three dimensional structure of the enzyme. The active site is a Zn2+ ion buried by 10E from the protein surface and is coordinated in an approximately tetrahedral geometry to four ligands azide, His94, His96, and His119. The azide nitrogen closest to the metal has a short contact to the hydroxyl oxygen of Thr199. Therefore it is natural to invoke the Thr199 as a group the modifies the potential energy function and controls the charges of the zinc bound azide. We plan on investigating the infrared photon echoes from a series of mutants of CA-II at the 199 position as a direct probe of the structural fluctuations surrounding this area, to gain some knowledge as to the degree of control this group has on the charge regulation in CA-II.
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